Fuels, such as liquid base coal slurries are difficult to ignite. The procedure usually followed is to inject a small quantity of pilot fuel, such as fuel oil, into the combustion chamber prior to injection of the coal slurry. The fuel oil is then ignited to promote ignition of the coal slurry.
The slurries are liquid based and must be injected at an early time during compression stroke to allow time for the liquid to evaporate. The evaporation of the liquid cools the air charge which increases the difficulty of igniting the pilot fuel. Accordingly, it is necessary to provide additional energy in the form of heat, either to the air or to the combustion chamber surfaces, to provide adequate heat to evaporate the liquid and to ignite the fuel charge.
Another problem encountered in attempting to burn difficult fuels, such as coal powder, has been that the powder may auto-ignite causing serious problems with engine timing. The timing problems may result in poor engine performance as well as possible damage to mechanical parts of the engine. With any of the coal fuels, severe ring and liner wear may occur when incomplete combustion is present and residual abrasive particles remain in the cylinder.
With the difficult fuels, a greater amount of heat is necessary in order to assure complete combustion. Due to the greater heat provided, the hot surfaces are necessarily heavily insulated and require special materials to prevent material failures in the hot areas, such as around the piston rings and the cylinder walls or liners.
In an effort to overcome such problems, design activity in the engine combustion area for promoting the burning of the difficult fuels has included the development of precombustion chambers wherein initial combustion occurs thus limiting the maximum heat to a confined area. Materials used in constructing such hot areas can be effectively utilized to avoid the above-mentioned problems.
U.S. Pat. No. 1,648,647 issued Nov. 8, 1927 to E. Kretigen illustrates the use of a precombustion chamber in an internal combustion engine of the diesel type. U.S. Pat. No. 1,696,799 issued Dec. 25, 1928 to G. Held illustrates the use of a specially designed combustion chamber to aid in the burning of heavy oils in engines of the diesel type.